1. Field of the Invention
The present invention generally relates to a liquid crystal medium useful as a material for an optical device, and in particular, to a liquid crystal medium having a liquid crystal phase over a wide temperature range, and having a large dielectric anisotropy and a large refractive index anisotropy. The present invention further relates to an optical device using the liquid crystal medium, and in particular, to an optical device that can be used in a wide temperature range and driven at a low voltage, and is capable of achieving a high-speed electro-optical response. The so-called optical device refers to various devices using electro-optic effect to achieve the function of optical modulation or optical switching, for example, display devices (LCD devices) and optical modulation devices used in optical communication systems, optical information processing or various sensor systems.
2. Description of Related Art
Liquid crystal display (LCD) devices utilizing liquid crystal compositions are widely used as the display for clocks, calculators, word processors, and so on. These LCD devices utilize the refractive index anisotropy and dielectric anisotropy and the like of liquid crystal compounds. The operation modes of the LCD devices are known and mainly include phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), and vertical alignment (VA) and so on, which utilize one or more polarizers for display. Moreover, in recent years, more attention has been paid to the mode where an electric field is applied to an optically isotropic liquid crystal phase to induce electric birefringence (Patent References 1-9, and Non-patent References 1-3).
Moreover, wavelength variable filters, wavefront control devices, liquid-crystal lenses, aberration correction devices, aperture control devices, and optical head devices and the like that utilize the electric birefringence in a blue phase as one of the optically isotropic liquid crystal phases have been proposed (Patent References 10-12).
According to the driving mode, the devices can be classified into passive matrix (PM) and active matrix (AM) types. The PM type is further classified into static type and multiplex type, and the AM type is further classified into thin film transistor (TFT) type and metal-insulator-metal (MIM) type.
Such LCD devices contain a liquid crystal composition having suitable physical properties. In order to improve the characteristics of an LCD device, the liquid crystal composition preferably has suitable physical properties. A liquid crystal compound as a component of the liquid crystal composition requires to have the following general properties:
(1) stable chemical properties and stable physical properties;
(2) a high clearing point (i.e., phase transition temperature from the liquid crystal phase to the isotropic phase);
(3) a low lower-limit temperature of a liquid crystal phase (i.e., optically isotropic liquid crystal phase, such as nematic phase, cholesteric phase, smectic phase, and blue phase);
(4) excellent compatibility with other liquid crystal compounds;
(5) a suitable dielectric anisotropy; and
(6) a suitable refractive index anisotropy.
Particularly, for an optically isotropic liquid crystal phase, a liquid crystal compound having a large dielectric anisotropy and a large refractive index anisotropy is preferred, in view of lowering the driving voltage.
When a liquid crystal composition containing a liquid crystal compound having stable chemical and physical properties as described in (1) is used in an LCD device, the voltage holding ratio is improved.
Further, a liquid crystal composition containing a liquid crystal compound having a high clearing point or a low lower-limit temperature of a liquid crystal phase as described in (2) and (3), may have an expanded temperature range of a nematic phase or optically isotropic liquid crystal phase, and thus can be used in a display device in a wider temperature range. A liquid crystal composition is generally prepared by mixing a liquid crystal compound with a number of other liquid crystal compounds, so as to exhibit better properties that are difficult to develop by a single liquid crystal compound. Therefore, a liquid crystal compound having good compatibility with other liquid crystal compounds as described in (4) is preferably used in an LCD device. In recent years, LCD devices with superior properties, especially display performance such as contrast, display capacity, and response time, are required. In addition, as for the liquid crystal material that is being used, a liquid crystal composition having a low driving voltage is required. Furthermore, in order to drive an optical device that is driven in an optically isotropic liquid crystal phase with a low voltage, a liquid crystal compound with a large dielectric anisotropy and a large refractive index anisotropy is preferred.
As for the optically isotropic polymer/liquid crystal composite materials disclosed in Patent References 1-3 and Non-patent References 1-3, high voltages for device operation are required. Although optically isotropic liquid crystal compositions and polymer/liquid crystal composite materials, which are expected to have an operation voltage lower than the voltages as described above, are disclosed in Patent References 4-9, an optically isotropic liquid crystal composition and a polymer/liquid crystal composite material of the present invention were not disclosed.